1. Field of the Invention
This invention relates to fiber optic sensors. The invention particularly relates to fiber optic sensors utilized for detecting the signature of a marine seismic source.
2. Description of the Prior Art
The use of optical fibers for detecting sound waves, temperature changes and other phenomena is well known. Fiber optic sensing systems include elements whose characteristics change with time, thereby introducing error into the system unless means are included for compensating for such changes. These changes include variations in the intensity of the optical source and variations in the light transmitting capacity of the optical paths.
U.S. Pat. No. 4,162,397, issued July 24, 1979 to Bucaro et al, shows an optical fiber acoustical sensor for detecting sound waves in a fluid medium. An optical fiber coil through which a light beam is transmitted is placed in a fluid medium. A sound wave propagating through the fluid medium and incident on the optical fiber coil changes the index of refraction of the optical fiber at the area of incidence. The change in the index of refraction causes a phase shift in the transmitted light which is detectable to denote the presence of the sound wave. This system, however, does not include any means of correcting for changes in the sensing system with time.
The article "Fiber Optic Thermometer Using Temperature Dependent Absorption, Broadband Detection, and Time Domain Referencing", by Grigory Adamovsky and Nancy D. Piltch, Applied Optics, Dec. 1, 1986, Vol. 24, No. 23, discloses a fiber optic thermometer. A reference channel is constructed in the time domain. A pulse modulated light source and a fiber-optic loop are utilized. The light pulse is coupled by a 2.times.2 coupler into a fiber optic loop which includes the temperature probe. A portion of the pulsed energy exits the loop when it returns to the 2.times.2 coupler and a portion travels around the loop again, and when it returns to the coupler, a second portion exits the loop. By comparing the amplitude of the energy exiting the loop after the first and second paths, the temperature is determined. Because the measurementss are made of two pulse amplitudes having the same origin, and traversing the same path, the system substantially compensates for time-varying parameters. A disadvantage is that the entire fiber optic recirculating loop, including the sensor, acts as a sensing element.
The article "Loss-Compensation Technique for Fiber-Optic Sensors and its Application to Displacement Measurements", by Glenn Beheim, Applied Optics, Feb. 1, 1987, Vol. 26, No. 3, discloses a system in which a beam splitting transducer modulates, as a function of the sensed parameter, the proportion of the incident light which it transmits and reflects. Using a four fiber optical link, light is impinged onto the transducer from either direction, and, in each case, the transmitted and reflected light are measured. These four signals are processed to remove the influence of the fiber and connector losses.